Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see reference 1]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
A known compound which has been shown to be an effective p-type semiconductor for organic FETs is bis(dithienothiophene) (BDT) [see reference 2-4]. When deposited as a thin film by vacuum deposition, it was shown to have carrier mobilities between 1×10−3 and 5×10−2 cm2V1 s−1 and very high current on/off ratios (up to 108). However, vacuum deposition is an expensive processing technique that is unsuitable for the fabrication of large-area films.
Regioregular poly(3-hexylthiophene) has been reported with charge carrier mobility between 1×10−5 and 4.5×10−2 cm2V−1 s−1, but with a rather low current on/off ratio (10-103) [see reference 5]. In general, poly(3-alkylthiophenes) show improved solubility and are able to be solution processed to fabricate large area films. However, poly(3-alkylthiophenes) have-relatively low ionisation potentials and are susceptible to doping in air [see reference 6].
It is the aim of the present invention to provide new materials for use as semiconductors or charge transport materials, which are easy to synthesize, have high charge mobility, good processibility and improved oxidative stability. Other aims of the invention are immediately evident to those skilled in the art from the following description.